In the process of a sulphate cellulose mill, chlorine (essentially present in the combined form, primarily as sodium chloride) accumlates in the chemical cycle as the amount of chlorine accumlates the process from the raw material, such as wood, from water, together with substituting chemicals, etc., is greater than the amount of chlorine removed from the process in any form. Even though chlorine is removed together with the pulp and at other process stages, it, however, tends to accumlate in the process. Excessive chlorine causes disadvantageous corrosion and pluggings in the soda recovery boiler and is also otherwise disadvantageous in the regeneration of chemicals. Various attempts have been made to remove chlorine, e.g. together with salts in the form of NaCl. U.S. Pat. No. 3,746,612 discloses a method for removing chlorine from white liquor, where white liquor is treated in an extremely complicated way in order that chlorine contained in the solution could be separated as NaCl. In general, the removal of chlorine from the process is difficult in most cases and requires plenty of extra equipment. In addition, chlorine removal processes are difficult to adjust.